Image forming apparatuses such as laser printers using an endless belt for conveying a paper or an endless belt for performing intermediate transfer have been conventionally known. Generally, such belt as a belt unit is detachably mounted on an apparatus main body so as to be replaced after use for a predetermined period, as disclosed in Japanese Patent Application Publication No. 2004-109267, for example. In such belt unit, at least two belt support rollers including a drive roller are supported by a belt frame and a belt is looped around the belt support rollers. A transfer member such as a transfer roller is also supported by the belt frame at a position in confrontation with a photosensitive drum disposed at the apparatus main body with the belt interposed therebetween. The transfer member is pressed against the photosensitive drum by urging means such as a spring supported by the belt frame.